A standard split-jaw lathe chuck has a chuck body rotatable about an axis and formed with a plurality of angularly spaced and radially extending inner guides and with a plurality of angularly spaced and radially extending outer guides. Respective inner jaw parts are radially displaceable in the inner guides and respective outer jaw parts projecting axially forwardly from the chuck body are radially displaceable in the outer guides. Means including at least one operator engageable with the inner jaw parts can jointly radially displace same in the chuck body in the respective inner guides.
Such an arrangement can be provided with individual coupling members such as described in my copending patent application 886,348 filed Mar. 14, 1978. Such coupling members are each displaceable between an axially advanced coupling position engaging both of the respective jaw parts and coupling same together for joint radial displacement and axially withdrawn decoupling positions for relative radial displacement of the respective parts. Such an arrangement has the considerable advantage that the axial stroke of the operator can be converted, with a considerable mechanical advantage, into a relatively short radial stroke of the inner jaw parts. The outer jaw parts are set so as to be spaced slightly outwardly of the workpiece or tool to be gripped by releasing of the respective coupling members, so that even though the jaw parts have a reduced stroke, they can be set to hold any workpiece whose dimensions lie within a wide range. Furthermore such an arrangement allows outer jaw parts to be exchanged in the manner of jaw blanks for the use of the chuck with custom-made jaw blanks adapted for particular purposes.
It is known to provide such a chuck with a so-called safety ring that prevents displacement of any of the coupling members into the decoupling position except when the inner jaw parts are locked against displacement. This feature avoids a serious accident that can happen when the chuck is rotated at high speed without one of its outer jaw parts being properly secured to the respective inner jaw part. Such a ring has, however, several disadvantages. First of all it is not often convenient to mount a single such ring on a given chuck. Furthermore such a system, as in U.S. Pat. No. 3,682,491, requires the user to decouple all of the inner and outer jaw parts simultaneously, so that one-by-one adjustment of the jaws is impossible. Thus it is impossible for the lathe operator to merely adjust a single one of the jaws without completely unchucking the workpiece.